SUMMARY There are fundamental gaps in our understanding of how cancer cells acquire DNA damage-tolerance and evade chemotherapy (termed ?chemoresistance?). The gaps in our knowledge of DNA damage tolerance (and how it differs between normal and neoplastic cells), limit our ability to kill tumors without causing side effect toxicities to normal healthy tissues. Our long-term goals are to solve the problem of how neoplastic cells tolerate therapy, and identify new molecular vulnerabilities that can be specifically targeted for improved treatment. The objective here is to define a novel tumor-specific mechanism of chemoresistance and to establish its tractability as a therapeutic target. Remarkably, we discovered that Melanoma Antigen-A4 (MAGE-A4, a Cancer/Testes Antigen or 'CTA') is an activating binding partner of the DNA repair protein RAD18 (an E3 ubiquitin ligase). MAGE-A4 is absent from all normal somatic cells but pathologically activates DNA repair in cancer cells. MAGE-A4 also activates the E3 ligase TRIM69 which confers resistance to mitotic spindle poisons. MAGE-A4 expression is associated with poor patient prognosis yet its potential impact on the responsiveness of tumors to chemotherapy in a physiological setting is untested. Based on exciting and compelling preliminary studies, we will test the central hypothesis that MAGE-A4 pathologically reprograms ubiquitin signaling in tumors to confer chemoresistance to genotoxins and spindle poisons. The rationale is that defining the contribution of MAGE-A4 to chemoresistance will allow extraordinarily specific therapeutic strategies that target a unique molecular vulnerability of neoplastic cells. The Specific Aims are: SA1 Define contribution of pathologically-activated DNA repair to chemoresistance in vivo. SA2 Establish chemical tractability of MAGE-A4/RAD18 as a therapeutic target. SA3 Mechanistically define MAGE-A4/TRIM69 functions in mitotic progression and resistance to spindle poisons. In SA1 We will use a new transgenic mouse and orthotopic lung cancer models to determine how MAGE-A4/RAD18 impacts responses to chemotherapy in vivo. In SA2 we will screen peptide phage display libraries to identify sequence motifs that bind MAGE-A4 and disrupt the MAGE-A4/RAD18 interaction. Bioactive MAGE-A4 inhibitor peptides will be tested for anti-neoplastic activity. In SA3 we will define a novel role of MAGE-A4 in regulating TRIM69 and conferring resistance to spindle poisons. We will mechanistically define the MAGE-A4/TRIM69 signaling pathway and establish its role in tolerance of therapeutic taxanes. These experiments will likely establish MAGE-A4 as a druggable target for ameliorating chemoresistance in cancer cells, serving as a crucial gateway in the drug discovery process. The proposed ideas and research are innovative because they are the first studies to test how biological activities of CTAs affect cancer therapy. The proposed work is significant because it will provide new paradigms for chemoresistance due to pathological ubiquitin signaling leading directly to novel targeted therapies for chemoresistant cancer.